Abstract

Unipore diffusion models are widely used for modelling gas transport in coal matrix in conventional dual-porosity coalbed reservoir simulators. The unipore models implemented in conventional coalbed reservoir simulators assume that free gas phase is negligible and gas exists only in adsorbed state in coal matrix. In low-rank coals, however, a substantial amount of free gas may exist in the macropores of coal matrix.

There is strong laboratory evidence that many coals exhibit bi- or multi-modal pore structure. This paper describes the implementation of a bidisperse pore-diffusion model in a coalbed reservoir simulator. In the bidisperse model, gas adsorption is assumed to take place only in the micropores, with the macropores providing storage for free gas, as well as tortuous paths for gas transport between the micropores and cleats. Gas production performance from a sub-bituminous Powder River Basin coalbed reservoir has been studied using the in-house Imperial College coalbed reservoir simulator. The implementation of the triple-porosity formulation in the simulator overcame the reported inconsistency between field gas production rates and predicted rates obtained using conventional dual-porosity simulators. With the introduction of an appropriate storage volume of free gas in the macropores, the predicted increase in gas production rates are consistent with the published field data.

Introduction

Coal seams may be characterised by two distinctive porosity systems: a well-defined and almost uniformly distributed network of natural fractures (cleats), and matrix blocks containing a highly heterogeneous porous structure between the cleats. The cleat system can be subdivided into the face cleat, which is continuous throughout the reservoir, and the butt cleat, which is discontinuous and terminates at intersections with the face cleat, Figure 1. The cleat spacing is very uniform and ranges from the order of millimetres to centimetres.

Unlike conventional gas reservoirs, methane in medium volatile bituminous coalbeds is primarily stored as a sorbed gas, at near liquid densities, on the internal surface area of the microporous coal. The surface area of the coal on which the methane is adsorbed is very large (20–200 m2/g)1 and, if saturated, coalbed methane reservoirs can have five times the volume of gas contained in a conventional sandstone gas reservoir of comparable size.

Virgin seams are often saturated with water. During primary recovery by pressure depletion, methane production is facilitated by dewatering the target seams to allow desorption of the adsorbed methane, which then migrates through the coal matrix into the cleats. The transport of gas through a coal seam is considered a two-step process. It is generally assumed that flow of gas and water through the cleats is laminar and obeys Darcy's law. On the other hand, gas transport through the porous coal matrix is controlled by diffusion2.

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